>I doubt anyone seriously believes NK could have manufactured the malware that was used.

It's actually quite possible. The Shamoon/Destover family borrows a lot of code from commercial products, with some standard command & control functionality added. This is not Stuxnet style malware: Stuxnet was designed to spread perniciously and send precise commands to SCADA controllers; this malware is designed to wipe drives and capture basic information and files at the behest of an operator.

Many groups could have made this malware, nation-state or otherwise, including NK.